This invention relates to an optical device for projecting light onto an aperture mask-panel assembly of a color cathode ray tube (CRT), in order to expose a photosensitive layer on the face of the panel to a pattern corresponding to a desired pattern of luminescent material (phosphor).
In the manufacture of color CRTs for color television, the three-color phosphor screen on the face of the CRT used to display the television image is conventionally formed photolithographically by exposing photosensitive layers on the face to a pattern of light corresponding to the desired pattern of phosphor on the screen.
The finished CRT includes three electron guns in the neck of the tube for directing three electron beams, one for each of the primary colors red, blue and green, to excite the phosphor elements on the screen. An aperture mask positioned a short distance behind the screen has apertures placed to allow the electron beams to impinge only upon the phosphor elements of the corresponding color. A full color display is produced when the three electron beams, each carrying a separate video signal for its primary color, are scanned across the screen. It will be appreciated that in order to achieve the desired color purity, accurate registration between the beams and the corresponding phosphor elements is required.
In order to achieve such accurate registration, the screen is formed by placing a light source at the approximate position of the source of one of the electron beams, and exposing the photosensitive material (resist) through the same aperture mask which will be used in later operation of the tube. Thus, once the screen has been exposed through the mask, the mask is said to be "married" to the panel.
The exposure insolubilizes the resist, after which the unexposed portions are washed away, leaving a first pattern of phosphor elements corresponding to a first primary color. The light source is then moved to correspond to the source of the second electron beam, and the process is repeated to produce a second pattern for the second color, and likewise for the third, resulting in a pattern of alternating red, green and blue phosphor elements in the finished screen.
Because the paths of the light rays during exposure are not identical to the paths of the scanned electron beams during tube operation, a correcting lens is placed between the source and the mask-panel assembly during exposure to achieve a light distribution more nearly like the distribution of the scanned electron beams. In addition, since the insoluble portion of the resist grows larger as the intensity of the exposure increases, a shader plate is also placed in the light path to achieve a desired distribution of light intensity across the face.
Another aspect of achieving accurate registration between the beams and the phosphor elements is the necessity of shielding the beams from external magnetic fields such as the earth's magnetic field during tube operation, which fields could otherwise result in noticeable deflection of the beams from their desired paths.
Such shielding is often accomplished by attaching an internal magnetic shield (IMS) to the mask frame. Such an IMS is typically fabricated from one or more sheets of a soft magnetic material such as steel into a shape which surrounds and extends rearward from the frame along the wall of the tube's envelope.
Since the frame supports the IMS, the IMS need not be self-supporting, and can therefore be relatively easily fabricated from thin gage material. However, such a construction creates handling and assembly problems. A more rugged construction offers the advantage of being readily transportable to an assembly area of the factory floor, where the proper size IMS can be selected and rapidly attached to the mask frame with less risk of damage than for a more fragile IMS.
Unfortunately, the construction of a more rugged IMS requires a heavier gage material and forming operations which work the material sufficiently to locally alter the magnetic properties of the material. Such non-uniformity of magnetic properties must be removed by annealing the IMS in order to prevent non-uniform shielding of the electron beams, which would cause registration errors in the tube. Such annealing adds significantly to the cost of the IMS.